Recent studies have shown that the hormonal form of vitamin D, 1,25-dihydroxyvitamin D3 (calcitriol), can affect both tissues and cells that are not directly involved in calcium homeostasis. In particular, a role for calcitriol as a regulator of immune cell differentiation and proliferation has been proposed. Specific high-affinity intracellular receptors for calcitriol (VDR) are detectable in activated T cells, and activated macrophages are able to synthesize calcitriol. A possible paracrine mechanism of action has been postulated. Vitamin D may therefore have a similar role to that of other immune regulatory molecules such as cytokines. The precise interaction of calcitriol with the cytokine network is not yet fully defined, but its ability to modulate immune cells in vitro and its association with inflammatory diseases are now well documented. These findings are outlined in this review with particular reference to effects on macrophages and lymphocytes. Ectopic production of calcitriol Initial evidence